Web offset printing and production requires precision bending of lithographic plates to assure precise alignment of the images on the plates and longevity of the plates. Plate runs can vary between twenty-five thousand to one million impressions without plate malfunction, depending in substantial part on the preparation and bending of the plate. Common problems with plates attributable to the web bending fixture utilized include plates with edges that are bent too sharply, not allowing the plate to seat naturally on the cylinder lips, plates with edges which are not bent sharply enough to fit the cylinder lips, plates which do not have parallel leading and trailing edge bends, plates that are out of register because not properly aligned prior to bending or not properly immobilized during bending, and plate lengths between bends which are too long or too short for the cylinder or plates having bent edges which are too long or too short for the cylinder because the plates were improperly positioned on the bender before bending.
Since the plate must conform precisely to the press cylinder on which it is to be utilized these problems cause the plate to be deformed when printing takes place and causes metal fatigue, normally in the area of the bends in the plate, which eventually leads to flexing, stress and eventual cracking of the plate. The actual physical formation of the bends in the plate is also of extreme importance because bending mechanisms which distort the metal when bending cause stress and stress fractures in the bend which substantially shortens the printing life of the plate. Imprecise bends, such as bends which are not straight, also lead to flexing of the plate during printing which substantially shortens the plate life.
Inventive activity has been directed to the bending of lithographic plates, most of which has incrementally improved various aspects of bending of such plates. For example, very precise bends have been achieved with bending fixtures employing the principles of a brake press where the material is clamped before bending and a bar or plate forms a precise bend at the edge of the clamp. Brake press bending, however, distorts and stretches the metal and changes the metal structure resulting in varying thicknesses of metal in the bend causing fractures and stress at the bend which shortens plate life. Other attempts have been made to resiliently form the metal around the edge of the anvil with the use of compression or tension springs. However, such springs are normally more resilient than the metal being bent and imprecise bending of the edges can occur, particularly in view of the bending pressures applied.